LiAlH additives

Furthermore, Fig. 3.35 shows the desorption cnrves of the ball-milled (MgH h-50 wt%LiAlH ) composite dnring continuous heating np to 250,260,275 and 300°C under 0.1 MPa hydrogen atmosphere. Under these conditions the composite desorbs 4.3 and 4.9 wt%H2 at 250 and 260°C, respectively. The pnrity-corrected amonnt of hydrogen which conld be desorbed from the 50 wt%LiAlH constitnent in a composite at these temperatnres which are higher than the temperatnres of the solid state reaction (Rib) of (3.12) and (R2) of (3.13) in Fig. 3.30b, is 3.8 wt%. Experimentally observed values are larger by about 0.5-1.0 wt% than the theoretical one. This excess could only be desorbed from MgH. That means that MgH is able to desorb at temperatnres 250 and 260°C, which are lower than its eqnilibrinm temperature of desorption nnder 0.1 MPa equal to 275°C. Apparently, MgH is thermodynamically destabilized by the second composite constituent LiAlH. Additional evidence that MgH is, indeed, destabilized is provided by the shape of the desorption cnrves at 250 and 260°C in Fig. 3.35 in which one can see a clearly discernible third... 

Caution. Butyllithium reacts violently with water and should be handled under an inert atmosphere. Until the ethanol quench, all manipulations are performed under anhydrous, oxygen-free conditions. To minimize chances for an explosion, diethyl ether should always be tested for peroxides prior to distillation from LiAlH . Additionally, under no circumstances should the distillation still be allowed to dry.5... 

The conversion of carboxylic acid derivatives (halides, esters and lactones, tertiary amides and lactams, nitriles) into aldehydes can be achieved with bulky aluminum hydrides (e.g. DIBAL = diisobutylaluminum hydride, lithium trialkoxyalanates). Simple addition of three equivalents of an alcohol to LiAlH, in THF solution produces those deactivated and selective reagents, e.g. lithium triisopropoxyalanate, LiAlH(OPr )j (J. Malek, 1972). 

The previous product was added to LiAlH (6 eq.) in THF. The solution was heated at reflux for 1 h. The excess hydride was destroyed by dropwise addition of water and the resulting mixture filtered through Celite. The filtrate was diluted with EtOAc, washed with brine and dried (Na2S04). The product was an oil (3.4 g, 98%). 

Preparation. Commercial manufacture of LiAlH uses the original synthetic method (44), ie, addition of a diethyl ether solution of aluminum chloride to a slurry of lithium hydride (Fig. 2). 

Andreasen et al. [86] also found that ball milling increased the rate constant, k, in the JMAK equation (Sect. 1.4.1), of reaction (Rib) in solid state but virtually had no effect on the rate constant of reaction (R2). They also showed that the reaction constant, k, of reaction (Rib) in solid state increases with decreasing grain size of ball-milled LiAlH within the range 150-50 mn. Andreasen et al. concluded that the reaction (Rib) in solid state is limited by a mass transfer process, e.g., long range atomic diffusion of Al while the reaction (R2) is limited by the intrinsic kinetics (too low a temperature of decomposition). In conclusion, one must say that ball milling alone is not sufficient to improve the kinetics of reaction (R2). A solution to improvement of the kinetics of reaction (R2) could be a suitable catalytic additive. 

The effect of catalytic metal chloride additives on the kinetics of isothermal decomposition of LiAlH in a Sieverts-type apparatus has been studied by a few research groups and the results seem to be rather contradictory. 

In another recent development Kojima et al. [103] mechanically milled LiH and Al without and with the TiClj additive for 24 h in a H gas atmosphere at a pressure of 1 MPa at room temperature. They found that a sm l amount of LiAlH could be directly synthesized by the mechanochemical reaction with concomitant formation of LijAlHg. The latter can be relatively easily formed by mechanochemical synthesis of LiAlH and LiH as originally reported by Zaluski et al. [71] and later by Balema et al. [104],... 

Figure 3.30 shows the DSC traces for the (MgH + 20, 30, 50 and 70 wt%LiAlH ) composites. Only single endothermic peak centered at 350°C is visible in DSC traces for the (MgH + 20 wt%LiAlH ) composite (Fig. 3.30a). This peak corresponds to the decomposition of MgH. The first low temperature exothermic effect observed in Fig. 3.9 for a pure LiAlH (both unmilled and milled), which is usually assigned to the interaction of LiAlH with hydroxyl impurities [67], is not observed in Fig. 3.30a-c but it appears in Fig. 3.30d for (MgH + 70 wt%LiAlH ). Four endothermic events occur for (MgH + 30, 50 and 70 wt%LiAlH ) (Fig. 3.30b-d). The first endothermic peak at 174-182°C has almost exactly the same temperature range as (Rla) in Fig. 3.9. No exothermic peak (Rib) of melting from Fig. 3.9 is seen in Fig. 3.30a-d. It seems that the addition of just 30 wt%MgH suppresses melting of LiAlH and its first decomposition into LijAlH and Al ((Rib) in Fig. 3.9) occurs from a solid phase and is endothermic. This is supported by the observation of partial decomposition of LiAlH into (LijAlH + Al) during milling as discussed before. The second endo peak in Fig. 3.30b-d at 198,193 and 223°C, respectively, corresponds to the decomposition...

Figure 3.32 shows XRD patterns of (MgH -i-LiAlH ) composites after DSC testing up to 500°C. The primary phases present are Mg and Al. Peaks of MgO and (LiOH) HjO arise from the exposure of Mg and Li (or possibly even some retained LiH) to the environment during XRD tests. Apparently, XRD phase analysis indicates that a nearly full decomposition of original MgH and LiAlH hydride phases has occurred to the elements during a DSC experiment. In addition, no diffraction peaks of any intermetallic compound are observed in those XRD patterns. That means that no intermetallic compound was formed upon thermal decomposition of composites in DSC. Therefore, the mechanism of destabilization through the formation of an intermediate intermetallic phases proposed by Vajo et al. [196-198] and discussed in the beginning of this section seems to be ruled out of hand. 

Fig. 3.36 (a) Desorption curves for the (MgHj + Xwt%Al) composites with the content of A1 additive equivalent to the content of free A1 formed after decomposition of Awt%LiAlH in (MgH + Xwt%LiAlH ) composites (300°C 0.1 MPa Hj). The (MgH + Xwt%Al) composites were baU milled for 20 h. (b) Dependence of an effective kinetic parameter, k, in the JMAK equation on the equivalent content of A1 metal additive... 

Dimethoxyamphetamine. A solution of the above nitropropene (17.0 g) in 500 ml of dry ether is added slowly to a stirred solution of lithium aluminum hydride (LiAlH or LAH) 12 g suspended in 150 ml of dry ether. After completing the addition, the mixture is refluxed for 20 hours, then cooled. The excess LAH is decomposed by careful addition of water. The resulting suspension is filtered and the solid removed is washed with ether. The combined ether solutions are dried with MgS04, and then saturated with dry hydrogen chloride. This precipitates the title compound which is filtered and recrystallized from ethanol. Yield 13 g, mp 111.5-112.5°. 

Synthesis of the 24R isomer was commenced by stereoselective hydroxymethy-lation of the enolate of lactone 30. Introduction of methyl groups at C25 and C26 was achieved by addition of MeLi to give 24R depresosterol (33). Alternatively, trapping of the lactone enolate with acetone followed by LiAlH reduction gave the 245 epimer (34). Spectral comparison indicated that the 24R sterol is identical with the natural product. 

It is also known, that in addition to the catalytic reaction route the dehalogenation of aromatic halides can be carried out in stoichiometric reaction [ 1] in the presence of metal hydrides (e.g. LiAlH or NaBH ). Further characteristic feature of hydrodehalogenation reactions is the requirement for addition of free bases into the reaction mixture to fix the formed hydrochloric acid [2,4]. 

Reduction with LiAlH(OBu )3293 or LAH292 also gives selective hydride addition to the less substituted allyl end (equations 317 and 318). In contrast, formate reductions selectively deliver hydride to the more substituted allyl terminus (equations 319 and 320).302-303 Si—H-mediated reduction, conveniently performed with polymethylhydrosiloxane (PMHS), demonstrates no clear pattern of regioselectivity (equation 321).320 LiHBEt3 delivers hydride regioselectivity to the less substituted allyl terminus (equation 322)289-291... 

Phosphine 1 shows the expected P NMR 6P-11.1 (THF), typical (11) of ortho substituted triphenylphosphine. Addition of LiAlH to this solution gives molecular hydrogen and two signals in the 31p NMR spectrum at -11.1 and -30.3. This observation suggests that deprotonation of 1 leads to a relatively slow equilibrium of phosphine alkoxide 5 (major component) and phosphoranide 6. This mixture and alkyl halides give phosphoranes such as 4 ... 

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